Conventionally, a trench gate structure in which a trench is formed in a semiconductor substrate and a gate electrode is buried in the trench is employed in semiconductor devices such as IGBTs (Insulated Gate Bipolar Transistor), MISFETs (Field Effect Transistor), and is advantageous in application to power sources and the like. For example, IGBTs having the trench gate structure have both a high-input impedance characteristic of a MISFET and a low saturation voltage characteristic of a bipolar transistor, and therefore, are utilized extensively in uninterruptible power source devices, various kinds of motor driving devices and the like.
FIG. 8 is a perspective view showing a semiconductor device having a conventional trench MOS gate structure disclosed in Japanese Patent Application Laid Open Publication No. 2003-017699A. As shown in the drawing, the conventional semiconductor device includes: a N-type base layer 101 formed in a silicon substrate; a P-type base layer 102 provided on the N-type base layer 101; a N-type source region 103 selectively formed in the P-type base layer 102; a gate insulating film 104 formed on the inner wall face of a trench T; a gate electrode 105 buried in the trench T; an insulating film 106 formed on the gate electrode 105 in the trench T; an emitter electrode 107 formed on the insulating film 106; a P-type collector layer 108 formed on the lower face of the N-type base layer 101; and a collector electrode 109 in contact with the P-type collector layer 108. Further, a P-type channel layer 112 is formed at a channel part on the side wall side of the trench T.
With the semiconductor device (MISFET) having the above trench MIS gate structure, reduction of ON resistance and high integration are enabled.
However, when the gate length is shortened in association of miniaturization of semiconductor integrated circuit devices, it is necessary to lower acceleration voltage for well implantation for minimizing the channel length and to reduce a dose amount at well implantation for restraining diffusion length from being longer. Also, it is necessary to further reduce the dose amount at well implantation for restraining an impact ionization phenomenon and for lowering the strength of an electric field to be applied to the gate insulating film at the bottom of the trench. Therefore, punch-through immunity is lowered and a controllable range of a threshold voltage Vt is restricted in the semiconductor device having the conventional trench gate structure, which are disadvantageous.